Continuing scientific and technological advances in the area of high-temperature superconducting ceramic (HTSC) materials are expected to have a significant economic impact in the next century. Numerous applications involving the use of these materials as thin and thick films, in wire form, and in bulk form are being explored. Applications based on HTSC thin films--e.g., filters for advanced communications systems, probe coils for magnetic resonance systems, and squids for non-destructive evaluation (NDE) and biomagnetism instruments--are progressing the most rapidly because thin films possess the necessary high current-carrying capacity. In contrast, applications requiring bulk superconductors have advanced at a slower pace because bulk materials having high critical current density are difficult to process. Only with significant increases in critical current density and the development of economically viable processing methods will the many potential applications of bulk HTSC materials be realized. These applications include bearings for superconducting motors, rails for magnetic levitation systems, components for high-efficiency generators, electromagnetic pumping, magnetic heat pumps, magnetic separation, and many other special-purpose devices. Early developmental work involving the use of bulk HTSC materials in an experimental generator, superconducting flywheel for long-term energy storage, and in high-temperature superconducting bearings have recently been reported.
For most applications requiring bulk HTSC material, the flux-trapping capacity is of primary importance in determining its utility, particularly when the material is to be used as a permanent magnet. To achieve a high flux-trapping capacity, the material must be able to carry a circulating current throughout the bulk form at high critical current density. In polycrystalline HTSC materials, achieving high critical current density is greatly inhibited by crystalline misalignments introduced at grain boundaries and the presence of other defects. For a given HTSC composition, the highest critical current density can thus be expected to occur in single crystals. The growth of high-quality superconducting single crystals, however, presents material processing challenges, both in fabricating required precursor materials and in optimizing the crystal growth process.
Specific targeted applications for these single crystals are superconducting clamping devices, electromagnetic superconductive dent pullers for use in the aircraft industry, bearings for flywheels, and advanced rivet guns. Even the simplest of these devices--an electromagnetic clamp--requires single crystals at least 3 cm in diameter able to trap a magnetic field of more than 1 T.
For most other applications, more stringent requirements must be met. Single crystals as large as 10 cm in diameter.times.2 cm thick and capable of trapping at least 3 T(J.sub.c &gt;2400 A/cm.sup.2) will be needed. Meeting this need will require significant advances in the materials and processes involved in producing HTSC single crystals, as well as an improved understanding of fundamental aspects of the overall crystal growth process.
Although the mechanisms for the peritectic reaction and growth of aligned grains in HTSC materials are well-documented, the growth of high-quality, large single crystals remains a challenging task. Early attempts to form bulk materials followed traditional ceramic processing methods such as the solid-state sintering of powder compacts containing appropriate mixtures of Y.sub.2 O.sub.3 CuO.sub.3 and BaCO.sub.3 Measured critical current densities were typically less than 1000 A/cm.sup.2 and dropped off sharply in the presence of small magnetic fields (&gt;0.5 T). Subsequent attempts involved the growth of 123 crystalline domains from a melt. As the superconducting 123 phase (YBa.sub.2 Cu.sub.3 O.sub.7) is heated above its peritectic melting temperature of about 1010.degree. C., it melts incongruently to form a non-superconducting 211 phase (Y.sub.2 BaCuO.sub.5) and a liquid phase rich in barium and copper. When the two-phase mixture is cooled below the peritectic temperature, the 211 phase and the liquid phase react to form 123: EQU Y.sub.2 BaCUO.sub.5 +3 BaCU.sub.2 +2 CuO!(liquid).fwdarw.YBa.sub.2 CU.sub.3 O.sub.7-x
Other crystal growth processes rely on solid state reactions between Lanthanide, Ba, and Cu oxides to produce the starting materials. The solid state reaction of oxide powders to requires several heating and grinding steps before phase pure powders are obtained. The powders in other works are also frequently mixed with platinum by ball milling or with a mortar and pestle. Platinum also has been introduced using the melt-powder-melt-growth technique.
A number of processes for producing bulk material have been explored based on this melting and reaction behavior. Reported approaches include the melt-textured growth (MTG) process, the quench-melt-growth (QMG) process, and the melt-powder-melt-growth (MPMG) process. The MTG approach does not yield large crystals. In the other two processes, the starting powders are first melted at temperatures above the peritectic temperature in platinum crucibles, cooled or quenched, and then reheated under various conditions to HTSC. Aligned polycrystalline structures typically result. Although such process variations have demonstrated some ability to influence phase formation and distribution in the resulting materials, none of these approaches has resulted in high critical current densities and trapped fields in large samples. For example, materials produced by the MTG method were reported to have critical current values of up to 17,000 A/cm.sup.2 in the absence of an applied magnetic field. Crystals as large as 8 cm in diameter have reportedly been grown by the MPMG process, and use of the process was claimed to be essential to such success.
Currently all single crystal growth processes slowly cool the sample, utilizing temperature and/or composition gradients to reduce the probability of nucleation away from the seed. While other processes require complex layering steps to make compacts with compositional gradients and/or the careful placement of a single sample in a specially designed furnace so that the temperature gradient in the furnace will align with the desired direction of growth in the sample.
Prior work has shown that high temperature ceramic superconductors as large as 8 cm in diameter can be grown by heating a pressed compact (usually in disk shape) of YBa.sub.2 Cu.sub.3 O.sub.7 (123) powder above the YBa.sub.2 Cu.sub.3 O.sub.7 .fwdarw.Y.sub.2 BaCuO(211)+Liquid peritectic temperature of 1010.degree. C. to achieve a homogeneous distribution of 211 particles within the liquid matrix and then nucleating the 123 crystal at a predetermined nucleation site of SmBa.sub.2 Cu.sub.3 O.sub.7 seed crystal while the sample is slowly cooled below 1010.degree. C. At a temperature range below 1010.degree. C., the crystal growth is initiated and completed while the samples are cooled through this range at a cooling rate of 0.01.degree. C./min. For a successful growth, platinum is added into the 123 powder compact in order to refine the 211 particles during the homogenization heat treatment above 1035.degree. C. and a temperature gradient is introduced into the compact in order to prevent the nucleation of 123 anywhere else other than the SmBa.sub.2 Cu.sub.3 O.sub.7 seed crystal.
Several problems associated with this process include: (i) the use of temperature gradient limits the number of samples that can be processed in a gradient furnace; (ii) continuous cooling limits the sample size to kinetically defined range; and (iii) samples lose considerable amount of liquid (&gt;25 wt %) as a result of chemical reactions with the substrate and consequently the crystal growth is arrested after a critical amount of liquid is lost.
Another problem in growing single crystals is the difficulty of removing some samples from the MgO substrates. This demonstrates the importance of a suitable setter powder. Without an effective setter powder, separating the sample from the substrate is not only a processing nuisance but there is also risk of damaging the sample. Furthermore, the inert setter powder should not act as a nucleation site for the sample. This is important in the processing of single crystals and in inhibiting the nucleation of crystals away from the primary seed. Better setter powders are needed to facilitate bulk HTSC growth.
The ceramic powders used to make the HTSC materials can have a dramatic effect on the resulting product. Traditional multi-component ceramic powder processing usually relies upon solid-state reactions to produce powders having the desired final phase(s). The overall process typically involves wet or dry mixing and grinding of constituent powders as oxides or carbonates, followed by an appropriate calcination step to form the desired material. Due to the nonoptimal distribution of individual constituents within the batch, the resulting powder may be non-homogeneous and contain undesirable secondary phases and/or unreacted components. Repeated mixing/grinding and calcining steps are frequently required to achieve acceptable homogenization and phase-pure powders.
Various alternative powder processing methods have been developed for producing ceramic powders having greater compositional uniformity. Many of these techniques also yield much finer particles, which facilitate reactions and densification during sintering into useful components. One of these techniques is called spray pyrolysis. It consists of a solution/precipitation process much more amenable to process control than traditional solid-state powder processing methods. A detailed review of ceramic powder synthesis using this general approach has been provided by Messing et al., J.Am. Ceram. Soc. 76, p.2702 (1993). The process allows for the production of particles having controlled morphology, composition and reproducibility provided process variables are well understood.